Name: ______________________________________
Date: ________________________
Student Exploration: Colligative Properties
Vocabulary: boiling point, colligative property, concentration, dissociate, freezing point,
manometer, osmosis, osmotic pressure, solute, solution, solvent, vapor pressure
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
1. When the forecast calls for freezing rain, salt is often spread on the roads. Why is this done?
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2. Antifreeze is added to car radiators to prevent the water in the radiators from both freezing
and boiling. How do you think antifreeze affects the temperatures at which water freezes
(the freezing point) and boils (the boiling point)?
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Gizmo Warm-up
Adding salt or other substances to water can affect the
temperature at which it freezes or boils. These effects and
others, known as colligative properties, are explored in the
Colligative Properties Gizmo™.
Check that No solute is selected and the Air temp is 25 °C.
1. Describe the motion of the water molecules in the beaker.
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2. Change the Air temp to -10 °C. What do you see as the water freezes? ________________
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3. Now change the Air temp to 110 °C. What do you see as the water boils? ______________
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Activity A:
Vapor pressure
and boiling point
Get the Gizmo ready:
 Check that No solute is selected.
 Set the Air temp to 25 °C.
Introduction: At the surface of a body of water, water molecules are constantly moving from
the liquid phase to the gas phase, and vice versa. The molecules that evaporate collide against
the walls of the beaker, creating vapor pressure.
Vapor pressure can be measured by a manometer, a liquid-filled tube that is connected to the
top of the beaker. Increasing the vapor pressure pushes the water in the tube, causing the water
in the right arm of the tube to rise to a higher level than the water in the left arm of the tube.
Question: How does salt affect the properties of water?
1. Observe: Gradually increase the Air temp to 95 °C. As you do this, observe the water in the
manometer. What do you notice, and what does this indicate about the vapor pressure?
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2. Record: Click the Record button below the table to see the vapor pressure in kilopascals
(kPa). Use the Gizmo to find the vapor pressure at 5 °C, 50 °C, and 95 °C.
5°C: ________
50 °C: ________
95 °C: ________
3. Analyze: How does the temperature of the liquid relate to the vapor pressure? ___________
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4. Measure: To measure the boiling point, increase the Air temp to 110 °C. When you see
bubbles, click Record. The water temperature at this time is equal to the boiling point.
A. What is the boiling point of pure water? ______________________
B. What is the current vapor pressure? ______________________
C. Atmospheric pressure at sea level is equal to 101.32 kilopascals (kPa). Why do you
think water boils when the vapor pressure is equal to atmospheric pressure?
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(Activity A continued on next page)
Activity A (continued from previous page)
5. Compare: Set the Air temp. to 25 °C, and click Clear. Compare the vapor pressure with No
solute and with Sodium chloride selected.
A. How does the presence of sodium chloride affect the vapor pressure of water?
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B. Based on this effect, how do you expect the presence of sodium chloride to affect the
boiling point of water? _________________________________________________
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6. Predict: How do you expect the addition of sodium chloride (table salt) to affect the freezing
point of water? _____________________________________________________________
7. Gather data: Fill in the first line of the table below. Then, use the Gizmo to determine the
boiling point and freezing point of the sodium chloride solution to complete the table.
Solution
Vapor pressure at 25 °C
Boiling point
Freezing point
Pure water
Water + sodium chloride
8. Analyze: How does adding sodium chloride affect the boiling and freezing points of water?
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9. Apply: Based on what you have learned, why do people spread salt on the roads in winter?
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10. Explore: Select the STUDY CONCENTRATION tab. On your own, explore how the
concentration of sodium chloride affects the colligative properties of water.
Describe your results: _______________________________________________________
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Activity B:
Get the Gizmo ready:
Dissociation
 Select the STUDY SOLUTE tab.
 Set the Air temp to 25 °C.
Introduction: When an ionic compound such as sodium chloride (NaCl) dissolves in water, it
dissociates, or breaks up, into individual Na+ and Cl– ions. Different ionic compounds may
dissociate into different numbers of ions, depending on their composition. Covalent compounds
such as sucrose do not dissociate.
Question: How does the dissociation of a solute affect the colligative properties of a
solution?
1. Observe: When the Show molecular view is on, you can see how many particles of solute
are dissolved in the solvent (water). Solute particles are colored yellow, dark blue, green,
and red. (In the molecular view, you see the result of dissolving two molecules of solute into
the solvent.)
List the number of each kind of particle below.
No solute: __________
Sucrose: __________
Calcium chloride: __________
Sodium chloride: __________
Potassium chloride: __________
2. Predict: Colligative properties are determined solely by the number of solute particles in the
solution. Based on this fact, which solute do you expect to have the greatest effect on the
properties of water? The least effect?
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3. Gather data: For each solute, measure the vapor pressure at a temperature of 25 °C. Then
measure the boiling point and freezing point of each solution.
Solute
Number of
particles
None
Sucrose
Sodium chloride
Calcium chloride
Potassium chloride
(Activity B continued on next page)
Vapor pressure
at 25 °C
Boiling point
Freezing point
Activity B (continued from previous page)
4. Analyze: How did the number of solute particles affect the colligative properties of water?
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5. Find a pattern: The chemical formulas of the three ionic solutes are NaCl (sodium chloride),
CaCl2 (calcium chloride), and KCl (potassium chloride).
A. How does the chemical formula relate to the number of particles that result from the
dissociation of a molecule? _____________________________________________
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B. Aluminum chloride has the formula AlCl3. How many particles would a molecule of
aluminum chloride dissociate into? _______________________________________
C. How many particles would two molecules of AlCl3 dissociate into? _______________
D. Based on the data you have collected so far, predict the boiling point and freezing
point of a 1.0-mol/kg solution of aluminum chloride: __________________________
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6. Check: Select the STUDY CONCENTRATION tab. You can model a 1.0-mol/kg solution of
aluminum chloride by using a 2.0-mol/kg solution of sodium chloride. Use the Gizmo to find
the boiling point and freezing point of a 1.0-mol/kg aluminum chloride solution.
Solute
Number of
particles
Vapor pressure
at 25 °C
Boiling point
Freezing point
Aluminum chloride
7. Think and discuss: Colligative properties depend only on the number of particles, not the
identity of the solute. What evidence could you use to demonstrate this point?
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Extension:
Osmotic pressure
Get the Gizmo ready:
 Select the STUDY SOLUTE tab.
 Set the Air temp to 25 °C.
Introduction: Osmosis is the flow of water across a semipermeable membrane from a region
of lower solute concentration to a region of higher solute concentration. The membrane allows
water, but not solute, to go through. Like boiling and freezing, osmosis is a colligative property.
Question: How do different solutes affect osmosis?
1. Measure: Adding pressure to a solution can prevent water from entering the solution via
osmosis. The osmotic pressure of the solution is equal to the pressure required to stop
osmosis. A higher osmotic pressure indicates a more highly saturated solution.
Select Sucrose and click Record. What is the osmotic pressure of sucrose? ____________
2. Predict: Based on the osmotic pressure of sucrose, predict what the osmotic pressures will
be for the other solutes:
Sodium chloride: _______
Calcium chloride: _______
Potassium chloride: _______
3. Gather data: Use the Gizmo to measure the osmotic pressure of each solute. (Note: The Air
temp must be set to 25 °C to collect data on osmotic pressure.) Write the results below.
Sodium chloride: _______
Calcium chloride: _______
Potassium chloride: _______
4. Analyze: How close were your predictions? ______________________________________
5. Apply: Eating a diet rich in sodium can increase the risk of high blood pressure. Why would
excessive amounts of sodium in the blood lead to higher blood pressure?
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6. Summarize: What do all the colligative properties you have studied have in common?
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